Tapered lensed fiber for focusing and condenser applications

ABSTRACT

A tapered lensed fiber includes a tapered multimode fiber having a gradient-index core and an optical fiber attached to the tapered multimode fiber. A method for forming a tapered lensed fiber includes attaching an optical fiber to a multimode fiber having a gradient-index core, applying heat to a surface of the multimode fiber, and pulling the multimode fiber into a taper. The method also allows for forming a tapered polarization-maintaining fiber while preserving stress rods and polarization isolation properties of the polarization-maintaining fiber.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from U.S. ProvisionalApplication Serial No. 60/298,841, entitled “Thermally Formed LensedFibers for Imaging and Condenser Applications,” filed Jun. 15, 2001 andU.S. Provisional Application Serial No. 60/352,735, “Tapered LensedFiber For Focusing and Condenser Applications”, filed Jan. 29, 2002.

BACKGROUND OF INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates generally to lensed fibers for deliveringand collecting light in optical communication networks. Morespecifically, the invention relates to a tapered lensed fiber forfocusing and condenser applications and a method for forming the taperedlensed fiber.

[0004] 2. Background Art

[0005] A lensed fiber is a monolithic device having an optical fiberterminated with a lens. Lensed fibers are advantageous because they areeasy to assemble, i.e., they do not require active fiber-lens alignmentand gluing of fiber to lens, they have low insertion loss, and theyenable component miniaturization because they can be made very small.The coefficient of thermal expansion of the lens can be matched to thatof the optical fiber to achieve better performance over a temperaturerange. Lensed fibers are easily arrayed and are therefore desirable formaking arrayed devices, for use in silicon optical bench applications,for aligning optical fibers to planar waveguides, and so forth. Inaddition, the spot size and working distance of the lensed fiber can betailored for a specific application. For example, the spot size andworking distance can be tailored to produce smaller beam diameters thatcan allow use of smaller micro-electro-mechanical-systems (MEMS) mirrorsin optical switches.

[0006] There are three main types of lensed fibers, includingcollimating lensed fibers, focusing lensed fibers, and tapered lensedfibers. FIG. 1 shows a prior-art collimating lensed fiber 20 having aplanoconvex lens 22 fusion-spliced to one end of an optical fiber 24.The optical fiber 24 may be a single-mode or multimode fiber. In theillustration, the optical fiber 24 is a single-mode fiber. Theplanoconvex lens 22 is formed from a coreless fiber, and the frontsurface of the lens 22 is shaped like a sphere. The spherical surface ofthe lens 22 is typically formed by melting the coreless fiber using anelectric arc or a laser beam. The spherical surface of the lens 22 actsas a collimator, expanding the light coming out of the optical fiber 24into a collimated beam. In practice, the lensed fiber 20 is used tocouple light from one fiber to another.

[0007]FIG. 2A shows a prior-art focusing lensed fiber 26 having a firstsquare-law index fiber 28 fusion-spliced to one end of a single-modefiber 30. A second square-law index fiber 32 is connected to the firstsquare-law fiber 28. A convex surface 34, which acts as a lens, may beprovided at a distal end 36 of the second square-law index fiber 32. Ingeneral, the radius of curvature of the convex surface 34 is smallerthan the radius of curvature of the lens for the collimating lensedfiber (see lens 22 in FIG. 1). The focusing lensed fiber 26 focuseslight into a spot and is useful for focusing applications, such asfocusing a beam onto a detector or receiver and coupling light from alaser source to an optical fiber.

[0008] There are various methods for providing the convex surface 34 atthe distal end 36 of the second square-law index fiber 32. One methodinvolves melting the distal end 36 to form a hemispherical surface. Asecond method involves chemically etching the distal end 36 to form theconvex surface 34. A third method, which is illustrated in FIG. 2B,involves fusion-splicing a coreless fiber 38 to the distal end 36 of thesecond square-law index fiber 32 and then melting the coreless fiber 38to form a hemispherical surface. Melting is usually based on electricdischarge.

[0009]FIG. 3 shows a prior-art tapered lensed fiber 40 having a taper 42formed at a tip of a single-mode fiber 44. The taper 42 has a convexsurface 46 that acts as a lens. The taper 42 can be achieved by grindingand/or polishing the tip of the single-mode fiber 44. The tapered lensedfiber collimates light over a short working distance. The tapered lensedfiber can be used for coupling light between an optical fiber and alaser source or an optical amplifier or a planar waveguide.

SUMMARY OF INVENTION

[0010] In one aspect, the invention relates to a tapered lensed fiberwhich comprises a tapered multimode fiber having a gradient-index coreand an optical fiber attached to the tapered multimode fiber.

[0011] In another aspect, the invention relates to a method for forminga tapered lensed fiber which comprises attaching an optical fiber to amultimode fiber having a gradient-index core, applying heat to a surfaceof the multimode fiber, and pulling the multimode fiber into a taper.

[0012] In another aspect, the invention relates to a method for forminga tapered polarization-maintaining fiber which comprises attaching apolarization-maintaining fiber to a multimode fiber having agradient-index core, applying heat to a surface of the multimode fiber,and pulling the multimode fiber into a taper.

[0013] In another aspect, the invention relates to a taperedpolarization-maintaining fiber which comprises a tapered multimode fiberhaving a gradient-index core and a polarization-maintaining fiberattached to the tapered multimode fiber.

[0014] Other features and advantages of the invention will be apparentfrom the following description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

[0015]FIG. 1 shows a prior art collimating lensed fiber.

[0016]FIGS. 2A and 2B show prior art focusing lensed fibers.

[0017]FIG. 3 shows a prior art tapered lensed fiber.

[0018]FIG. 4 shows a tapered lensed fiber according to an embodiment ofthe invention.

[0019]FIG. 5A shows a multimode fiber spliced to a single mode fiber.

[0020]FIG. 5B shows the multimode fiber of FIG. 5A being tapered.

[0021]FIG. 6 shows mode field diameter as a function of distance fromlens surface for a tapered lensed fiber formed according to anembodiment of the invention.

[0022]FIG. 7 shows far field divergence angle as a function of radius ofcurvature of taper for tapered lensed fibers formed according to anembodiment of the invention.

DETAILED DESCRIPTION

[0023] Embodiments of the invention provide a tapered lensed fiber forfocusing and condenser applications and a method for fabricating thetapered lensed fiber. In general, the tapered lensed fiber includes amultimode fiber spliced to any single-mode fiber, including apolarization-maintaining (PM) fiber. A PM fiber propagates only onepolarization. The multimode fiber has a gradient-index (GRIN) core,i.e., the refractive index of the core changes according to apredetermined profile. The multimode fiber is pulled to create a taper.Typically, the pulling process is accompanied by heating of themultimode fiber. The lens effect of the tapered fiber comes from bothrefraction and gradient index in the taper. Therefore, there is moreflexibility in tailoring the mode field diameter (MFD) and divergenceangle of the lensed fiber for a specific application, which ultimatelyleads to improved coupling efficiency. The method of the inventionallows tapered PM fibers to be formed while preserving stress rods andpolarization isolation properties of the PM fibers. Specific embodimentsof the invention are described below with reference to the accompanyingfigures.

[0024]FIG. 4 shows a tapered lensed fiber 2 according to one embodimentof the invention. The tapered lensed fiber 2 includes a taperedmultimode fiber 4 attached to an optical fiber 6. The multimode fiber 4can be attached to the optical fiber 6 by any suitable means, e.g., byfusion-splicing or by an index-matched epoxy. The tapered multimodefiber 4 has a core 8 surrounded by a cladding 10. The core 8 has agradient index in that its refractive index varies according to apredetermined profile, where the profile is determined by the targetapplication. Methods for making multimode fibers with GRIN cores areknown in the art. Typically, the gradient refractive index is achievedby introducing dopants into different layers of the glass material thatforms the core. The optical fiber 6 also has a core 12 which issurrounded by a cladding 14. The optical fiber 6 could be anysingle-mode fiber, including PM fiber.

[0025]FIGS. 5A and 5B illustrate a method for forming the tapered lensedfiber (2 in FIG. 4). In accordance with this method, a multimode fiber5, which will become the tapered multimode fiber 4 in FIG. 4, isfusion-spliced to the optical fiber 6. The process for fusion-splicingthe fibers 5, 6 involves placing the terminal ends 5 a, 6 a of thefibers 5, 6 in abutting position, as shown in FIG. 5A, and then heatingthe ends 5 a, 6 a while pushing them together, as shown by the arrows.Any suitable source of heat, such as resistive heating, electric arc, orlaser beam, can be used to fuse the ends 5 a, 6 a together. Once thefibers 5 and 6 are fusion-spliced together, the multimode fiber 5 isthen subject to a tapering step.

[0026] As shown in FIG. 5B, the tapering step involves moving a heatsource 7 along the multimode fiber 5 while pulling the multimode fiber 5and optical fiber 6 in opposite directions, along their longitudinalaxes, as indicated by the arrows. The multimode fiber 5 elongates as itis pulled. Preferably, the heat source 7 is a resistive filament. Oneadvantage of using a resistive filament is better control of the taperangle and symmetry. An example of a resistive filament suitable for usein the invention is a tungsten filament loop included in a fusionsplicer sold under the trade name FFS-2000 by Vytran Corporation ofMorganville, N.J. However, it should be clear that the invention is notlimited to this specific resistive filament or to resistive heating. Forexample, the heat source 7 could also be an electric arc or a laserbeam.

[0027] When the tapering process is complete, the multimode fiber 5 willlook like the multimode fiber 4 in FIG. 4. The tapered multimode fiber 4shown in FIG. 4 acts as a lens, where the lens effect comes from therefraction and gradient index in the tapered multimode fiber 4.Typically, the length of the tapered multimode fiber 4 is about 125 μmor greater. As shown in FIG. 4, the tip 16 of the tapered multimodefiber 4 has a radius of curvature. Typically, the radius of curvature issmall, e.g., about 5 to 30 μm. The resistive filament (7 in FIG. 5B)used in forming the tapered multimode fiber 4 allows for formation of aspherical tip (or lens) 16 with a symmetrical mode field. The radius ofcurvature of the tip 16 of the tapered multimode fiber 4 can be adjustedby controlling the power supplied to the filament (7 in FIG. 5B). Ingeneral, the higher the power supplied to the filament (7 in FIG. 5B),the larger the radius of curvature.

[0028] It should be noted that the method of forming a tapered lensedfiber described above allows for the combination of tapered fiberfeatures with PM fibers. Ordinarily, if a PM fiber is pulled into ataper, stress rods in the PM fiber will be destroyed. If stress rods aredestroyed, the polarization isolation properties of the PM fiber willnot be maintained. In the present invention, the polarization isolationproperties of the PM fiber can be maintained by splicing a multimodefiber to the PM fiber and then subjecting the multimode fiber to atapering process.

[0029] In operation, a light beam transmitted through the tapered lensedfiber 2 is focused into a spot upon exiting the tapered multimode fiber4. In general, the larger the radius of curvature of the tip 16 of thetapered multimode fiber 4, the larger the spot size.

[0030] The following example is intended for illustration purpose onlyand is not to be construed as limiting the invention as otherwisedescribed herein.

[0031]FIG. 6 shows mode field diameter along the x and y axes (see FIG.4) as a function of distance along the z-axis (see FIG. 4) for a taperedlensed fiber having a multimode fiber with an outer diameter of 125 μmand a GRIN core of diameter 62.5 μm fusion-spliced to a single-modefiber with a 9-μm core. The zero point on the z-axis (see FIG. 4) wasestimated from divergence angle in the lens of the beam emerging at thesplice formed between the multimode fiber and the single-mode fiber. Thebeam measurements were taken with a beam scan using 10 times objectiveat a numerical aperture of 0.17.

[0032]FIG. 7 shows angular radiation intensity as a function of radiusof curvature of tapers formed at the end of multimode fibers. Themultimode fibers had a 125-μm outer diameter and a 62.5-μm GRIN core.The radiation intensity was measured in far field by scanning from +72to −72 degrees using a goniometric radiometer LD 8900, available fromPhoton Inc. The graph contains data for both a single-mode fiber and apolarization-maintaining fiber fusion spliced to the multimode fiber.The single-mode fiber was a Corning® SMF-28 fiber having a 9-μm core anda 10.4-μm mode field diameter. Measurements were made using a broadbanderbium amplified spontaneous emission laser source. The graph shows adependence between divergence angle and radius of curvature of thetaper. The far-field divergence angle at 1/e² power level (θ) and themode field radius at beam waist (w₀) of the taper at the waist can berelated using θ=λ/(πw₀).

[0033] The multimode fibers were tapered at a length of about 300 μm andthen rounded to the desired radius of curvature. In general, the lengthof the GRIN region decreases as the radius of curvature increases. At asmall radius of curvature, the divergence angle increases withdecreasing radius of curvature, as is expected for a typical taperedfiber. However at a radius of curvature of about 16 μm, the divergenceangle starts to level off. At a radius of curvature greater than 16 μm,the divergence angle increases again. The divergence increases becausethe GRIN region becomes shorter with larger radius of curvature.

[0034] The invention provides one or more advantages. A tapered lensedfiber can be formed by fusion-splicing a single-mode fiber to amultimode fiber and then tapering the multimode fiber. Using this samemethod, a tapered PM fiber that maintains its polarization isolationproperties can be formed. The tapered lensed fiber of the invention canbe used for a variety of applications. For example, the tapered lensedfiber can be used to couple light from a single-mode fiber into asemiconductor optical amplifier or planar waveguide or other opticaldevice. Also, the tapered lensed fiber can be used to couple light inthe opposite direction, i.e., from a semiconductor optical amplifier ora planar waveguide or a spherical laser source or other optical deviceto a single-mode fiber.

[0035] While the invention has been described with respect to a limitednumber of embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

What is claimed is:
 1. A tapered lensed fiber, comprising: a taperedmultimode fiber having a gradient-index core; and an optical fiberattached to the tapered multimode fiber.
 2. The tapered lensed fiber ofclaim 1, wherein the tapered multimode fiber is terminated with a radiusof curvature.
 3. The tapered lensed fiber of claim 2, wherein the radiusof curvature ranges from about 5 to 30 μm.
 4. The tapered lensed fiberof claim 1, wherein the optical fiber is a single-mode fiber.
 5. Thetapered lensed fiber of claim 4, wherein the single-mode fiber is apolarization-maintaining fiber.
 6. A method for forming a tapered lensedfiber, comprising: attaching an optical fiber to a multimode fiberhaving a gradient-index core; applying heat to a surface of themultimode fiber; and pulling the multimode fiber into a taper.
 7. Themethod of claim 6, wherein applying heat to the surface of the multimodefiber comprises using a resistive filament to provide the heat.
 8. Themethod of claim 7, wherein applying heat to the surface 6 f themultimode fiber further comprises moving the resistive filament alongthe surface of the multimode fiber during heating.
 9. The method ofclaim 6, wherein applying heat to the surface of the multimode fiber andpulling the multimode fiber into a taper occur simultaneously.
 10. Themethod of claim 6, wherein pulling the multimode fiber into a tapercomprises forming a radius of curvature at a distal end of the taper.11. The method of claim 10, wherein the radius of curvature ranges fromabout 5 to 30 μm.
 12. The method of claim 6, wherein attaching themultimode fiber to the optical fiber comprises fusion-splicing themultimode fiber to the optical fiber.
 13. The method of claim 6, whereinpulling the multimode fiber into a taper comprises simultaneouslypulling the optical fiber and the multimode fiber in opposite directionsalong a longitudinal axis of the multimode fiber.
 14. The method ofclaim 6, wherein the optical fiber is a single-mode fiber.
 15. Themethod of claim 14, wherein the single-mode fiber is apolarization-maintaining fiber.
 16. A method for forming a taperedpolarization-maintaining fiber, comprising: attaching apolarization-maintaining fiber to a multimode fiber having agradient-index core; applying heat to a surface of the multimode fiber;and pulling the multimode fiber into a taper.
 17. The method of claim16, wherein applying heat to the surface of the multimode fibercomprises using a resistive filament to provide the heat.
 18. The methodof claim 16, wherein applying heat to the surface of the multimode fiberand pulling the multimode fiber into a taper occur simultaneously. 19.The method of claim 16, wherein pulling the multimode fiber into a tapercomprises forming a radius of curvature at a distal end of the taper.20. The method of claim 16, wherein attaching thepolarization-maintaining fiber to a multimode fiber comprisesfusion-splicing the polarization-maintaining fiber to the multimodefiber.
 21. A tapered polarization-maintaining fiber, comprising: atapered multimode fiber having a gradient-index core; and apolarization-maintaining fiber attached to the tapered multimode fiber.22. The tapered polarization-maintaining fiber of claim 21, wherein themultimode fiber is terminated with a radius of curvature.
 23. Thetapered polarization-maintaining fiber of claim 22, wherein the radiusof curvature ranges from about 5 to 30 μm.